The invention relates to a method for the patterned metallization of the surfaces of substrates, especially printed circuit boards, using electromagnetic radiation.
Published German Patent Application No. DE 3,922,478 discloses a method of this general kind. It describes the patterning of copper-coated polymer surfaces. Removal of a plastic coating and a remaining copper layer from between the conductors is performed by means of an excimer laser. It has been found, however, that the removal of a relatively thick copper layer by means of an excimer laser entails the production of heat, to such an extent that the edge sharpness necessary for patterning cannot be achieved. This signifies that a fine or superfine pattern, such as is ever more frequently desired, cannot be produced on such surfaces by means of an excimer laser. Furthermore, the removal of a relatively thick copper layer, such as a copper overlay, by means of an excimer laser is very time-consuming, so that a process of this kind was not practicable. Furthermore, if after removal of the spider by an excimer laser a galvanic conductor build-up is performed and followed by a differential etching process, it is impossible to achieve a fine or superfine edge on the conductors. Sharp edges of the necessary fineness are not achievable by the differential etching method since in this case undercutting of the conductors is unavoidable.
In particular, even the use of an etchant, which is necessary in the known processes, is not good from the environmental point of view. This is also true of the known method of disposing photosensitive layers on circuit boards wherein exposure to a light pattern causes a reaction which, after stripping, permits the conductors to be etched with a mordant.
In the periodical, Galvano Technik, 84 (1993), No. 2, pages 570 ff., in the article, "Herstellung flexibler Schaltungen mit Bayprint--eine zukunftsweisende Technologie" [production of flexible circuits with Bayprint, a pioneering technology], a process is described which operates without exposure to light and without etching. A polymer surface is imprinted with a primer, i.e., an adhesion-promoting coating, by the screen-printing method. The conductor patterns are here already established by the primer. The primer is nonconductive, adheres well to the polymer surface and, after a drying procedure, has on its surface a porous pattern which then permits the tight adhesion of a metal coating deposited on it without electric current. The metal coating then forms the conductor pattern. This process does have the advantage that it is relatively easy to perform; the disadvantage, however, is that the screen printing process is limited as regards the achievable fineness of the conductor pattern. The method described is not suitable, therefore, for the creation of very fine conductor webs of the kind ever more frequently sought. In this article, on page 571, first paragraph, it had already been proposed to provide polyimide materials with holes by penetration with an excimer laser. This makes use of the fact, known in itself, that polyimide is a very homogeneous material which can easily be ablated by the high photon energy of an excimer laser.